In the world of flight, weight is everything. In nature, birds have bones that are hollow, with criss-crossing struts or trusses for structural strength. The number of hollow bones varies among species, though large gliding and soaring birds tend to have the most. Some birds even have the uncanny ability to vomit a recently eaten meal to distract a predator, with the bonus of shedding weight for a quick escape.
To the outsider, the balance of weight vs flight, might seem straight forward; “more power!!!” as Jeremy Clarkson would say. But in reality, the guys of Top Gear would be all-too-familiar with how cyclical the battle can be. Adding extra power to an aircraft requires a larger engine or two, which also requires more fuel, adding weight. So how can the capacity of an aircraft be improved? The secret is minimising the weight of the aircraft itself.
But first things first, why is weight so important for an aircraft? The answer is simple, the less weight, the more likely you’ll get airborne. An overloaded aircraft chews through fuel at an increased rate and may fail to take-off, have a decreased angle and rate of climb (reducing obstacle clearance capability) and suffer from impaired controllability, manoeuvrability and increased stall speeds. Reduced braking effectiveness, landings requiring a longer stretch of runway and compromised structural strengths are also consequences of an overweight aircraft.
When a manufacturer designs an aircraft and a governing body certifies it, the aircraft’s limitations in terms of structural integrity and maximum loading capacity are calculated by measuring the surface area of the wing, and how much lift it produces at a safe airspeed. For example, if a smaller GA aircraft requires a vast amount of speed to support an excessive load on the airframe, it would be deemed unsafe, as the excessive speed levels would compromise safety as well as efficiency.
Certified aircraft types receive a weight report delivered as part of their mandatory aircraft records, identifying the empty weight of the aircraft as well as its maximum take-off weight or MTOW. The MTOW is a fixed value that indicates the heaviest weight at which the aircraft type has been shown to meet all relevant airworthiness requirements. The MTOW combines operating empty weight, passenger and baggage weight, cargo, reserve fuel, fuel for the journey and taxi fuel into a single figure, usually measured in kilograms or pounds. Due to the structural limitations of aircraft, the MTOW cannot be exceeded. However, there are other weight considerations that are relevant for determining the aircraft’s safest gross weight – even when operating beneath its MTOW.
Variations when calculating the safest gross weight can include the maximum ramp weight (the heaviest weight an aircraft can be loaded while it is on the ground), the maximum zero fuel weight (the heaviest weight an aircraft can be loaded without having any usable fuel in the fuel tanks) and maximum landing weight (the heaviest weight an aircraft can have when it lands). The empty weight of the aircraft, which is the baseline for all weight calculation, includes all operating equipment that has a fixed location and is installed in the aircraft. Empty weight is the combined weight of the airframe, power plant, required equipment, optional or special equipment, fixed ballast, hydraulic fluid, and residual fuel and oil.
While different circumstances necessitate differing degrees of weight on an airframe, the bottom line is that the less the empty weight of an aircraft, the more capacity there is for fuel and payload. Engineers, designers and manufactures are constantly trying to minimise the empty weight of aircraft to allow for increased carrying capacity without compromising safety or efficiency. With modern advances in technology, particularly in the field material science, there have been significant developments that have given experts a significant edge in the battle.
One of the most prominent developments in aviation engineering technology is the progress of materials that enable the construction of aircraft that are lighter, without compromising safety. Over 100 years ago, the Wright brothers took to the air in a delicate wooden contraption they had built from little more than wood. They covered the wings with a cotton muslin fabric used predominantly for women’s underwear and they reinforced delicate areas of the aircraft with ash wood. The metal fittings were made from mild steel and the aircraft was rigged with 15-gauge bicycle spoke wire. The engine block was cast from a hard-aluminium alloy, and other parts of the engine were made from steel or cast iron, except for the spark points which contained tiny bits of platinum. With an empty weight of 274.4 kg and an MTOW of 338 kg, you can imagine how little wiggle-room either Orville or Wilbur had once on board for a picnic blanket and packed lunch.
Over the course of aviation, pioneering engineers and manufactures shifted from wood and fabric to metal but the problem of minimising weight endured. As the latest and potentially most significant aircraft material since aluminium alloys were created in the 1920’s, composites are giving aluminium a run for its money. Technological breakthroughs in material sciences and engineering mean that composite materials can be used to successfully improve the structural design of aircraft without the addition of unnecessary weight, which means more capacity for fuel or payload.
As the name suggests, composite materials are a composite of ingredients that result in a hybrid material. In a basic composite, one material acts as a supporting matrix, while another material builds on the matrix to reinforce the entire material. A base material matrix is laid out in a mould under high temperature and pressure and an epoxy or resin is poured over the base, bonding with it to create a strong and flexible material.
The first use of modern composite materials in aircraft was about 30 years ago when boron-reinforced epoxy composite was used for the skins of the empennages of the USAF F-14 and F-15 fighters. Initially, composite materials were used only in secondary structures, but as understanding of the materials improved over time, their use in structures such as wings and fuselages increased. The Beechcraft Starship, known for its canard design, was in 1989 the only civilian aircraft certified by the FAA to use carbon fibre composite so extensively. More recently, Jabiru, Diamond and Cirrus are capitalising on the advantages of composite materials, reducing weight and increasing fuel efficiency. The development of composite technology also allows for greater aerodynamics, adding greater precision to winglet and sharklet design which reduces drag and spares the weight of carrying more fuel on board.
Fibreglass is currently the most common composite material, and consists of glass fibres embedded in the resin matrix. Fibreglass was used widely in the 1950s for boats and cars and was first introduced to the aviation industry with the arrival of the Boeing 707 passenger jet in the 1950s, where it comprised about two percent of the overall structure of the aircraft. Each generation of new aircraft built by Boeing has had an increased percentage of composite material usage; the highest being 50% composite usage in the upcoming 787 Dreamliner – achieving a reported 20% reduction overall in empty weight.
Weight reduction is the greatest advantage of composite material usage but other benefits include high corrosion resistance and resistance to fatigue damage, both of which reduce operating costs. But along with the significant advantages to using a composite material construction, there are also considerable downsides. Being relatively new, the cost of using composite materials in aviation construction is high, a cost that is compounded by the labour intensive and often complex fabrication process. Even though it is heavier, aluminium, by comparison, is easier to manufacture and repair. Aluminium can also withstand dents or punctures, holding together despite a compromised structure. Composites, if they are damaged, require immediate repair, which is difficult and costly.
In some cases, especially when subjected to cyclic stress, the layers of the composite material can separate from each other or “delaminate”, compromising the integrity of the aircraft’s construction. Evidence of delamination is often invisible on the outer layers of composite materials, so newer and more expensive non-destructive testing methods are required to root out potential problems. However, with the increasing usage of composite materials, the availability of testing tools, more advanced repair methods and production costs will inevitably become more economical and continuing research and development points to a promising future for aerospace engineering.
But composite materials can’t hog all the limelight in modern engineering, not with the rise of 3D printing technology. 3D printing is an exciting development in general. Who doesn’t want the technology to design and ‘print’ any 3D shape you can think of quickly, cheaply and in vast numbers? Recent advances have even made 3D printed food a possibility, so you can create precision-made treats from cake batter to noodles. Want a cutaway Lockheed Super Constellation birthday cake complete with Qantas livery, passengers and crew? It’s definitely an intriguing possibility…
Anyway, 3D printing is a development with exciting possibilities, especially in an industry where every gram counts. In basic terms, 3D printing is the process of making a solid 3-dimensional object from a computerised model. Essentially, a design is created using an AutoCAD program which is sent to the 3D printing machine. The machine will then print layers of material or use lasers to fuse microscopic materials together, creating the object.
Last year at the 2016 Berlin Air Show, Airbus unveiled the first ever aircraft to be made using 3D printing. Dubbed ‘THOR’ (an acronym for Testing High-tech Objectives in Reality) the aircraft weighs in at just 21kg and measures less than four metres in length. Powered by two 1.5 kW electrically-driven propellers, 90 per cent of its structural components were 3D-printed from plastic polyamide powder. Okay, so it probably won’t be ferrying anything heavier than mice around for the time being, but Airbus says that it’s using the model as a platform to develop 3D printed technologies for aircraft in the future. According to an Airbus media release, it took approximately seven weeks to print the 60 structural segments that make up THOR with one week for assembly and three days to install electrical systems.
The quick turnaround of the model contributes to the streamlined testing of new aviation technology, explains Detlev Konigorski of Airbus’ Emerging Technologies & Concepts activity in Germany. “…[THOR] is a platform to enable low-risk and fast-track development of different technologies in real flying conditions… If a THOR aircraft takes off, and after 30 feet makes a nose dive back (to) the ground, our attitude is: ‘good, let’s sweep it off the runway and come up with a better idea…In a few weeks, we can print a new aircraft!”
But there are current 3D technologies being utilised in life-sized aircraft too. A case study from the Northwestern University, led by Professor Eric Masanet, used current aircraft industry data to examine the life-cycle environmental effects of using 3D printing for select metal aircraft parts, a technique that is already being adopted by the industry. Masanet’s team concluded that 3D-printing lighter and higher performance parts could decrease the cost of production while also decreasing the weight of the finished airplane.
As Professor Masanet states in the paper, “3D printing has been increasingly adopted by aircraft component manufacturers for lightweight cost-effective design… the aircraft industry has adopted several different AM [additive manufacturing – another term for 3D printing] components for reducing aircraft mass – including flight deck monitor arms, seat buckles, and various hinges and brackets – which can lead to greater fuel efficiency”.
In 2016 GE aviation introduced the first 3D printed parts to be used in an aircraft engine platform. According to the press release, each of the new CFM LEAP engines – produced jointly by GE and its long-time partner, Snecma (SAFRAN) of France – will have 19 3D-printed fuel nozzles in the combustion system. The 3D printed parts are 25% lighter than their predecessor part and are simpler in construction, reducing the number of parts used to construct the nozzle from 18 to 1. The 3D printed nozzle will also incorporate more intricate cooling pathways and support ligaments, resulting in five times more durability than conventional manufacturing. Closer to home, distributors like Corvus Aero are keeping a beady eye on the weight of their imported Italian engines, ever conscious of the need to provide maximum power via an engine that’s easy on the scales.
As well as reducing the weight of the parts themselves, 3D printing can cut down production time and waste. By placing material only where it is required, 3D printing reduces the amount of waste by eliminating the need to slough shapes out of a solid block. This efficiency translates to more cost-effective production which is passed onto the buyer. It also reduces the cost of maintenance and repairs as worn or broken components can be replaced much faster and at less cost to a manufacturer. And while printing whole aircraft may be a ways away yet, the momentum of 3D printing technology is set to impact all facets of aviation in the future.
In the cockpit, manufacturers and distributors are minimising the cost and weight of avionics by offering pilots the option of choosing individual avionics parts that specifically suits their needs. Manufacturers also reduce the cost to the buyer by offering avionics layouts with the addition of optional extras. Techam’s P2002 Sierra MKII, provides pilots with the choice of several avionics outfits on top of the standard analogue instrumentation from Garmin GPS system to the Dynon SkyView with twin 10 inch displays, the Garmin G3X featuring twin 10.6 inch screens and the G3X Night version with fully backlit backup instrumentation. Modern real estate for the cockpit that won’t strain the foundations.
While Glass cockpits and modern computerised avionics are lighter and offer more information in less space than analogue dials and meters, choosing the avionics that suit an aircraft’s role rather than ‘over building’ for potential roles minimises cockpit clutter. But streamlined avionics and other electrical components also help reduce the amount of cables and wiring required, something that can quickly start contributing to those precious few kilograms on board.
Electrical cables provide everything from power, data, sensor information, flight management control, avionics, and communications to overhead and emergency lighting and inflight entertainment. One option to reduce the weight of wiring and cables is to use lighter materials that can withstand higher temperatures so that less insulator or conductor is needed. However, according to a white paper release in June 2015 by Nexans, some 30% of all electrical wires in the A380-800 can potentially be replaced with a wireless system. According to model, size, passenger capacity and specifications, an airliner can contain anywhere from 200 to 600 km of cables interconnecting equipment throughout the aircraft, so the conversion isn’t insignificant.
Not only does it de-clutter and de-tangle the extensive amount of cables in the cockpit, wireless technology can prove to be a real weight saver from general aviation up to airliners. Rather than adding to the weight with the addition of countless Ethernet cables and USB ports, robust Wi-Fi could provide many passengers with a single source of internet.
On any level, being able to forgo in-built entertainment systems for personal tablets and mobile phones is another opportunity to reduce overall weight. Including an entertainment system in any business or commercial jet assumes that a passenger will plug-in to a screen fitted into the aircraft in some capacity. But if that passenger chooses to fill-in time by reading a book or having a snooze instead, that extra weight proves completely unnecessary. However, personal entertainment systems will also require technology that will transform an aircraft into a flying LAN (Local Area Network) hub – a development that will need significant improvements in broadband data capacity, or high-end cable solutions requiring optical fibre to replace the copper network already in place. No doubt this technology progresses it will also translate to general aviation.
And speaking of small things that quickly accumulate in mass, according to a media release in 2015, NASA and Boeing invested two weeks in Shreveport, Louisiana, testing non-stick wing coatings designed to shed insect residue and help reduce aircraft fuel consumption. While the question of a dead bug on the wing of an aircraft affecting the weight of an airliner sounds like the ponderings of a philosopher, the surprising fact is that the addition of a hitch-hiking bug can actually affect the efficiency of an aircraft.
“Laminar aircraft wings are designed to be aerodynamically efficient,” said Mia Siochi, senior materials scientist at NASA’s Langley Research Center in Hampton, Virginia. “If you have bugs accumulating, it causes the airflow to trip from smooth or laminar to turbulent, causing additional drag. An aircraft that’s designed to have laminar wings flying long distance can save five to six percent in fuel usage. Surprisingly, all you need are little bugs that trip the flow and you lose part of this benefit.”
Research involved studying the chemistry of bugs to gain insight into what happens when an airborne insect and a high-velocity aircraft meet in the sky. “We learned when a bug hits and its body ruptures the blood starts undergoing some chemical changes to make it stickier,” said Siochi.
Then the materials scientists studied lotus leaves, using the plant as a basis for creating the right combination of chemicals and surface roughness in the test coatings. “When you look at a lotus leaf under the microscope the reason water doesn’t stick to it is because it has these rough features that are pointy,” added Siochi. “When liquid sits on the microscopically-rough leaf surface, the surface tension keeps it from spreading out, so it rolls off. We’re trying to use that principle in combination with chemistry to prevent bugs from sticking.”
Of the numerous coatings trialled on the wings of the Boeing 757 ecoDemonstrator (a Boeing 757 modified for testing aviation technologies) two were considered to make enough of an impact and NASA plans to make them available for licencing by private companies. The technology will no doubt be of significant use to all areas of aviation, however the coating material to date has only blocked about 40 percent of bug splats, leaving room for more testing to ditch the free-loading insects once and for all.
Cutting-edge science is being constantly applied in aviation in the ceaseless quest to shed a few pounds and give our aircraft those beach-fit waistlines and airframes. Who knows what the future might bring?